We have found that estrogenic steroids and the adrenal steroid, dehydroepiandrosterone, protect cultured rodent cells against DMBA- and aflatoxin Bl-induced cytotoxicity and malignant transformation, and also inhibit metabolism of H3 DMBA to water-soluble products. Our observations are consistent with certain findings in humans: 1. Burch et al. have reported a much reduced incidence of cancer in a group of 737 hysterectomized women on long-term estrogen replacement therapy. 2. Several investigators have obtained evidence that women with subnormal plasma levels of dehydroepiandrosterone may be predisposed to develop breast cancer. Others have shown that estradiol-17 beta is a competitive inhibitor of benzo(a)-pyrene hydroxylase, and our hypothesis is that this steroid may protect in our system by competitively inhibiting carcinogen activation. Several investigators have found that dehydroepiandrosterone is a potent non-competitive inhibitor of glucose-6-phosphate dehydrogenase. Inhibition of this enzyme would reduce the extra-mitochondrial level of NADPH, a necessary cofactor for the metabolic activation of DMBA and aflatoxin Bl. Our immediate objective is to determine if dehydroepiandrosterone inhibits carcinogen activation by reducing the extra-mitochondrial level of NADPH. This will be done by measuring carcinogen activation in vitro in the presence of either a glucose-6-phosphate dehydrogenase or isocitrate dehydrogenase NADPH-generating system, and determining if dehydroepiandrosterone inhibits carcinogen activation only in the presence of the former NADPH-generating system.